If dest is not NULL, the wcstombs() function converts the wide-
character string src to a multibyte string starting at dest. At most
n bytes are written to dest. The sequence of characters placed in
dest begins in the initial shift state. The conversion can stop for
three reasons:
1. A wide character has been encountered that can not be represented
as a multibyte sequence (according to the current locale). In
this case, (size_t) -1 is returned.
2. The length limit forces a stop. In this case, the number of bytes
written to dest is returned, but the shift state at this point is
lost.
3. The wide-character string has been completely converted, including
the terminating null wide character (L'\0'). In this case, the
conversion ends in the initial shift state. The number of bytes
written to dest, excluding the terminating null byte ('\0'), is
returned.
The programmer must ensure that there is room for at least n bytes at
dest.
If dest is NULL, n is ignored, and the conversion proceeds as above,
except that the converted bytes are not written out to memory, and no
length limit exists.
In order to avoid the case 2 above, the programmer should make sure n
is greater than or equal to wcstombs(NULL,src,0)+1.

The wcstombs() function returns the number of bytes that make up the
converted part of a multibyte sequence, not including the terminating
null byte. If a wide character was encountered which could not be
converted, (size_t) -1 is returned.

This page is part of release 5.00 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2019-03-06 WCSTOMBS(3)